I just repaired an old fender twin? output transformer. A customer gave this to me years ago, I've used it as a paperweight but today I thought I'd take a peek at it. So one side of the primary was dead, nothing,zero resistance. i thought i'd open it up just in case. After cutting the paper winding coating I found the short. The winding, about 3cms from where the winding is attached to the primary wires, had shorted to the case, the silver case on the outside. Quite an arc, it's probably 6-7mm from the winding to the case in that location. About 1 cm of winding had converted into fire, leaving soot traces on the case and on that area of the core. I should have taken an image then but I got too excited and fixed it nice and quick. If you take a read good look at the first image, you can actually see the stub of the winding going back into the core wrap, just above the solder work. If you take a look at the last image you can see just how thin the primary winding of this transformer is, real thin. So now the primaries measure as they should, about 38 ohms on one side and 39 ohms on the other . I'll clean the rest of the soot off carefully and coat the area in nice thick epoxy to discourage any further shorts either to the case, or to the laminations.